Field-Effect Transistors Having Black Phosphorus Channel and Methods of Making the Same

ABSTRACT

Various transistors, such as field-effect transistors, and methods of fabricating the transistors are disclosed herein. An exemplary transistor includes a phosphorene-containing layer having a channel region, a source region, and a drain region defined therein. A passivation layer is disposed over the phosphorene-containing layer. A source contact and a drain contact extend through the passivation layer, such that the source contact and the drain contact are respectively coupled with the source region and the drain region. A gate stack is disposed over the channel region. In some embodiments, the gate stack includes a gate dielectric layer and a gate electrode layer, where the gate dielectric layer extends through the passivation layer and contacts the channel region. In some embodiments, the gate stack includes a gate electrode layer disposed over the passivation layer, and a portion of the passivation layer serves as a gate dielectric layer of the gate stack.

This application is a continuation application of U.S. patentapplication Ser. No. 15/477,279, filed Apr. 3, 2017, which is adivisional application of U.S. patent application Ser. No. 14/969,813,filed Dec. 15, 2015, now U.S. Pat. No. 9,620,627, the entire disclosuresof which are incorporated herein by reference.

BACKGROUND

A metal-oxide-semiconductor field-effect transistor (MOSFET), or simplya field-effect transistor (FET) or a transistor, is widely used inintegrated circuits (ICs), including digital integrated circuits, radiofrequency (RF) analog circuits, etc. The gate length of a transistor isreduced or scaled down to increase the packing density of transistors inan IC and to increase the speed performance thereof. However,transistors with aggressively miniaturized gate lengths suffer fromundesirable short-channel effects, such as increased off-state leakagecurrent.

To effectively suppress short-channel effects in transistors having gatelengths smaller than about 20 nanometers (nm), one way is to employ asemiconducting channel with reduced thickness, e.g. less than 5 nm. Atransistor having a semiconductor channel with a thickness less thanone-third or even one-quarter of its gate length is commonly known as anultra-thin body transistor. Ultra-thin body transistors may employultra-thin semiconductor channel materials. One type of ultra-thinsemiconductor materials with high mobility is black phosphorus (BP). BPis a layered material. A monolayer of BP, termed “phosphorene,” is asemiconductor with a direct band gap of about 2 eV at the F point of thefirst Brillouin zone. When a number of layers of phosphorene arestacked, the multi-layer BP has a reduced band gap, with the band gapreduced to about 0.3 eV for bulk BP.

However, existing methods of fabricating transistors with BP channelsuffer from various problems. One problem is that many of thesetransistors were realized using mechanically exfoliated BP layers. It isdifficult to employ the mechanical exfoliation technique to mass producetransistors with BP channels for large-scale ICs.

Another problem is the exposure of the BP layers to air or moisture inthe ambient prior to gate dielectric formation. Upon exposure to air,the surface of a BP layer becomes oxidized, and adsorbs moisture fromthe air. This irreversibly converts the BP surface to phosphorus oxide(PO_(x)) compounds, resulting in non-uniform degradation of the surfaceand/or edge properties of the BP layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detaileddescription when read with the accompanying figures. It is emphasizedthat, in accordance with the standard practice in the industry, variousfeatures are not drawn to scale and are used for illustration purposesonly. In fact, the dimensions of the various features may be arbitrarilyincreased or reduced for clarity of discussion.

FIG. 1A is a perspective view of a planar FET having a BP channel,constructed according to various aspects of the present disclosure.

FIG. 1B is a perspective view of a multi-gate FET having fin-like BPchannels, constructed according to various aspects of the presentdisclosure.

FIG. 1C is a cross-sectional view of the devices in FIGS. 1A and 1B,according to some embodiments.

FIGS. 2A and 2B show a flow chart of a method of fabricating thesemiconductor devices of FIGS. 1A and 1B, according to some embodiments.

FIGS. 3, 4, 5, 6, 7B, 8, 9, 10, 11, 12, 13, 14, 15, 16, and 17 arecross-sectional views of forming a semiconductor device according to themethod of FIGS. 2A and 2B, in accordance with some embodiments.

FIG. 7A is a perspective view of a device having multiple BP activeregions.

FIGS. 18 and 19 are schematic views of manufacturing tools forfabricating the semiconductor devices of FIGS. 1A and 1B, according tosome embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

The present disclosure is generally related to semiconductor devices.More particularly, it is related to semiconductor devices having a blackphosphorus (BP) layer for field-effect transistor (FET) channels and themethods of making the same. The term “BP layer” as used herein refers toone layer of phosphorene or multiple layers of phosphorene. An object ofthe present disclosure is to prevent non-uniform degradation of thesurface and/or edge properties of a BP layer during fabrication.According to some embodiments, the surface and/or edges of the BP layerare passivated throughout the process flow during the fabrication.

FIG. 1A shows a perspective view of a FET 100A having a planar BPchannel, and FIG. 1B shows a perspective view of a FET 100B having afin-like BP channel. Both devices 100A and 100B are constructedaccording to various aspects of the present disclosure. FIG. 1C shows across-sectional view of the FET 100A (along the “1-1” line of FIG. 1A)and the FET 100B (along the “2-2” line of FIG. 1B) in the same figurefor the purpose of simplicity, where the label “102(103)” in FIG. 1C andother figures refers to the feature “102” in FIG. 1A and the feature“103” in FIG. 1B collectively.

Referring to FIGS. 1A and 1C collectively, the FET 100A includes asubstrate 102, a BP layer 104 over the substrate 102, a passivationlayer 106 over the BP layer 104, a gate stack 108, and source and drain(S/D) contacts 116.

In embodiments, the substrate 102 includes a layer of insulatingmaterial and the BP layer 104 is formed on the surface of the insulatinglayer. In an embodiment, the insulating layer comprises silicon oxide.In another embodiment, the insulating layer comprises a crystallineoxide such as aluminum oxide.

The BP layer 104 includes one or multiple phosphorene, and provides twosource and drain (S/D) regions 104 a and a channel region 104 b betweenthe two S/D regions 104 a. The orientation of the source-to-draindirection of the FET 100A is in the direction of the light effectivemass of the phosphorene (the “x” direction), while the transversein-plane direction is in the direction of the heavy effective mass ofthe phosphorene (the “y” direction).

The passivation layer 106 is in direct contact with the BP layer 104. Inthe present embodiment, the passivation layer 106 provides multipleopenings through which the gate stack 108 and the S/D contacts 116 comein direct contact with the BP layer 104 at the channel region 104 b andthe S/D regions 104 a respectively.

The gate stack 108 includes a gate dielectric layer 110 and a gateelectrode 112 over the gate dielectric layer 110. The gate dielectriclayer 110 and the passivation layer 106 may be of the same or differentmaterial. In an alternative embodiment, the gate stack 108 includes thegate electrode 112 but does not include the gate dielectric layer 110.To further this embodiment, a portion of the passivation layer 106directly underneath the gate electrode 112 is not removed and functionsas the gate dielectric layer.

In the present embodiment, the FET 100A further includes a gate spacer114 on opposite sides of the gate stack 108, and an inter-layerdielectric (ILD) layer 118 over the passivation layer 106 and fillingthe spaces among the features 108, 114, and 116. In an alternativeembodiment, the FET 100A does not include the gate spacer 114. Thechannel region 104 b is directly underneath the gate dielectric layer110 while the other portions of the BP layer 104 are the S/D regions 104a including S/D extensions underneath the gate spacer 114.

Referring to FIGS. 1B and 1C collectively, similar to the FET 100A, theFET 100B also includes a substrate 102, a BP layer 104 over thesubstrate 102, a passivation layer 106 over the BP layer 104, a gatestack 108, source and drain (S/D) contacts 116, a gate spacer 114, andan ILD layer 118. One difference between the FETs 100A and 100B is thatthe substrate 102 of the FET 100B provides a non-planar insulatingsurface on which the various features are formed. In the presentembodiment, the non-planar insulating surface comprises a plurality offins or bars 103. The fins 103 include a dielectric material, such assilicon oxide. The BP layer 104 and the passivation layer 106 are formedconformally over the fins 103. The gate stack 108 and the S/D contacts116 engage multiple surfaces (e.g., the top and two sidewall surfaces)of the BP layer 104, forming a multi-gate transistor. A fin spacer 114′is disposed over sidewalls of the passivation layer 106. The fin spacer114′ includes the same material(s) as the gate spacer 114.

In various embodiments, each of the FETs 100A and 100B may be anintermediate device fabricated during processing of an integratedcircuit (IC), or a portion thereof, that may comprise static randomaccess memory (SRAM) and/or other logic circuits, passive componentssuch as resistors, capacitors, and inductors, and active components suchas p-type FETs, n-type FETs, metal-oxide semiconductor field effecttransistors (MOSFET), complementary metal-oxide semiconductor (CMOS)transistors, bipolar transistors, high voltage transistors, highfrequency transistors, other memory cells, and combinations thereof.More details about the FETs 100A and 100B, including the materials usedtherein and the methods of forming the same, are collectively describedbelow.

FIGS. 2A and 2B show a flow chart of a method 200 of forming atransistor having a BP channel, such as the FETs 100A and 100B,according to various aspects of the present disclosure. The method 200is merely an example, and is not intended to limit the presentdisclosure beyond what is explicitly recited in the claims. Additionaloperations can be provided before, during, and after the method 200, andsome operations described can be replaced, eliminated, or moved aroundfor additional embodiments of the method. The method 200 is discussed inconjunction with FIGS. 3-7B and 8-17 which depict cross-sectional viewsof the FETs 100A and 100B collectively, and FIG. 7A which depicts aperspective view of the FET 100A. The label 100A/B in the figures refersto both the FET 100A and the FET 100B.

At operation 202, the method 200 (FIG. 2A) provides a substrate 102.Referring to FIG. 3, in embodiments, the substrate 102 is an insulatoror a wafer. In an embodiment, the substrate 102 includes a dielectriclayer that provides a planar top surface. The dielectric layer maycomprise silicon oxide (SiO₂) or another crystalline oxide such asaluminum oxide (Al₂O₃). The dielectric layer may also comprise anamorphous dielectric material. In another embodiment, the dielectriclayer provides a topography such as fins (or bars) 103 (FIG. 1B). Thefins 103 may comprise a dielectric or insulating material which may beamorphous or crystalline in nature, such as SiO₂, Al₂O₃, or any otherdielectric materials. The fins 103 may be parallel to each other, asillustrated in FIG. 1B, or may be perpendicular, or neither parallel orperpendicular to each other. In the present embodiment, the fins 103 areparallel to each other. The width of the fins 103 may be less than 10nm, such as less than 5 nm.

At operation 204, the method 200 (FIG. 2A) forms a BP layer 104 over thesubstrate 102, which includes the fins 103 in some embodiments.Referring to FIG. 4, The BP layer 104 can be formed by varioustechniques. For example, orthorhombic black phosphorus can be grown by ashort-way transport reaction from red phosphorus and tin/tin-iodide as amineralization additive. In another example, black phosphorus can besynthesized from white phosphorus under high pressure (13,000 kg/cm²) ata temperature of about 200 degrees Celsius. The thickness of the BPlayer 104 can be controlled by the deposition time. In embodiments, thethickness of the BP layer 104 may range from 1 monolayer to 20monolayers (about 10.6 nm), such as from 1 monolayer to 10 monolayers(about 5.3 nm). The period (or spacing) between two layers of the blackphosphorus is about 0.53 nm.

The BP layer 104 may be formed over the entire substrate 102 orselectively on parts of the substrate 102. If the BP layer 104 is formedover the entire substrate 102, it may be separated into differentregions for forming different devices. This is performed in a later stepto be discussed. The BP layer 104 is formed either as a planar layer asshown in FIG. 1A or conformally on the surfaces of the fins 103 as shownin FIG. 1B.

At operation 206, the method 200 (FIG. 2) forms a passivation layer 106over the BP layer 104. Referring to FIG. 5, the passivation layer 106covers the entire surface of the BP layer 104, either as a planar layeror a conformal layer over the topography of the substrate 102. In thepresent embodiment, the passivation layer 106 is formed without breakingvacuum subsequent to the formation of the BP layer 104. This is toprevent the BP layer 104 from being exposed to excessive moisture andambient air. In various embodiments, partial pressures of the moistureand oxygen are reduced to 0.1 torr or lower during the operations 204and 206.

In an embodiment, operations 204 and 206 are performed within amulti-chamber tool such as a multi-chamber tool 300 in FIG. 18 or amulti-chamber tool 400 in FIG. 19. Referring to FIG. 18, the tool 300includes a black phosphorus deposition chamber 304 and a blackphosphorus passivation chamber 306. The BP deposition chamber 304 andthe BP passivation chamber 306 share one load port 302. Once the device100A/B is loaded into the tool 300 through the load port 302, it isprocessed in multiple chambers before it is unloaded (transferred out)from the tool 300. A transfer module 301 (e.g., a mechanical arm or arobot) is operable to move the device 100A/B among the chambers.Throughout the multi-chamber processing, a general vacuum environment ismaintained in the tool 300. For example, after the BP layer 104 isformed in the chamber 304, the transfer module 301 moves the device100A/B from the chamber 304 to the chamber 306 without breaking vacuum.Then, the passivation layer 106 is formed over the BP layer 104 in thechamber 306. Referring to FIG. 19, the tool 400 also includes a BPdeposition chamber 304 and a BP passivation chamber 306 as discussedabove. The tool 400 further includes a passivation layer etch chamber308, a black phosphorus etch chamber 310, and a metal or dielectricdeposition chamber 312. The functions of the various chambers will bedescribed later. The chambers 304, 306, 308, 310, and 312 share one loadport 302. A transfer module 301 is operable to move the device 100A/Bamong the chambers without breaking vacuum. In embodiments, the tools300 and 400 may each include multiple load ports 302 and multipletransfer modules 301, for example, to be able to process multiple wafersat the same time. Various other embodiments of multi-chamber tools canbe used by the method 200.

In an embodiment, operation 206 includes depositing an ultra-thin filmof metal over the BP layer 104 without breaking vacuum subsequent to theformation of the BP layer 104, and then oxidizing the film of metal. Themetal film can be formed by physical vapor deposition (PVD) orsputtering. To further this embodiment, the BP passivation chamber 306may comprise a vacuum compartment defined by chamber walls, a pedestalfor holding the device 100A/B, and a PVD target comprising the materialto be sputtered. A processing gas such as argon may be supplied to thechamber 306 at a low pressure (maintained by a vacuum pump), and meteredby a mass flow controller. A DC power supply may be used to negativelybias the PVD target with respect to the device 100A/B. Radio frequencycoil power may be inductively coupled into the chamber 306 by aninductive coil to create a plasma. The ionized gas impinges on thetarget with sufficient energy to dislodge atoms or multi-atom particlesfor deposition on the device 100A/B. In embodiments, the metal film isdeposited to a thickness of about 0.5 nm or greater. When the device100A/B comprising the metal film on the BP layer 104 is taken out of themulti-chamber tool 300 (or the tool 400), the metal film issacrificially oxidized in air and becomes the passivation layer 106,without oxidizing the BP layer 104. In an embodiment, the metal filmcomprises aluminum. But this is not limiting. In general, the metal maybe one that is easily oxidized in air, such as hafnium (Hf) andzirconium (Zr). Further, the metal should not react spontaneously withphosphorene at the deposition temperature, which may range from 10 to250 degrees Celsius. In another embodiment, the metal film may beoxidized under controlled conditions, such as in a chamber with oxygen,oxygen-containing gas, or water vapor. The chamber that oxidizes themetal layer may be in the same tool as the chamber 306.

In another embodiment, operation 206 includes depositing a dielectriclayer as the passivation layer 106 without breaking vacuum subsequent tothe formation of the BP layer 104. In a further embodiment, thepassivation layer 106 comprises an aluminum oxide (Al₂O₃) layer formedby atomic layer deposition (ALD). In a further alternative embodiment,the passivation layer 106 may comprise a dielectric material such ashafnium oxide (HfO₂), lanthanum oxide (La₂O₃), silicon oxide (SiO₂),boron nitride (BN), and zirconium oxide (ZrO₂). In these embodiments,the BP passivation chamber 306 is an ALD chamber and makes use of theself-limiting nature of surface reactions for thin film deposition. Forexample, to grow an Al₂O₃ passivation layer on the BP layer 104,alternating exposures of trimethlyaluminum (TMA) and H₂O is performed inthe chamber 306. In each cycle of TMA and H₂O exposure, a monolayer ofAl₂O₃ of about 0.11 to 0.12 nm is deposited. Before the cycling of TMAand H₂O, there may be multiple pulses of TMA and purge to ensure that alayer of Al first forms on the BP layer 104. The ALD may be performed attemperatures ranging from 150 to 350 degrees Celsius, for example.

In yet another embodiment, the passivation layer 106 comprises asemiconductor such as silicon, germanium, carbon, or their alloys, suchas silicon-germanium-carbon. To further this embodiment, operation 206includes depositing the semiconductor material over the BP layer 104without breaking vacuum subsequent to the formation of the BP layer 104.The deposition may be a chemical vapor deposition using precursors suchas SiH₄, Si₂H₆, GeH₄, Ge₂H₆, and CH₄.

After operation 206, the device 100A/B comprises the BP layer 104covered by the passivation layer 106. The two layers may cover theentire surface of the substrate 102, such as a wafer or portionsthereof. In an embodiment where the layers 104 and 106 cover the entiresurface of the substrate 102, the method 200 may further include anoperation 208 of separating the BP layer 104 (and the passivation layer106 thereon) into multiple active regions (or BP active regions) so thattransistors, such as FETs 100A and/or 100B, may be formed in each activeregion. This is illustrated in FIG. 6 where two active regions 101 areformed by operation 208. Each of the active regions 101 includes aportion of the BP layer 104 underneath a portion of the passivationlayer 106. In an embodiment, operation 208 includes one or morelithography processes followed by an etching process to define aplurality of active regions or islands of BP layers. The portions of theBP layer 104 and the passivation layer 106 outside the defined activeregions are removed by the etching process. Each active region 101 maybe further processed into one or more transistors after the etching.Some active regions 101 may be dummy regions for the purpose of ensuringgood etching uniformity. The etching process may be a wet etch or a dryetch. The wet etch may comprise etching in water (H₂O) or dilutedhydrofluoric acid (HF). The dry etch may be performed using anoxygen-containing plasma, or simply using water vapor. The reaction ratemay be adjusted by tuning the temperature, pressure, and oxygenconcentration. The dry etch may be performed in one or more etchchambers of a multi-chamber tool, such as the passivation layer etchchamber 308 and the BP etch chamber 310 of FIG. 19.

After the BP layer 104 and the passivation layer 106 are etched, someedges (sidewall surfaces in the “x-z” or “y-z” plane) of the BP layer104 are no longer covered by the passivation layer 106. Operation 208further includes an edge passivation process to cover those edges with apassivation layer 106A as shown in FIGS. 7A and 7B. FIG. 7A illustratesa perspective view of the device 100A, while FIG. 7B illustrates across-sectional view of the device 100A along the 3-3 line of FIG. 7A.Although a perspective view of the device 100B at this fabrication stageis not illustrated, person having ordinary skill in the art can deriveit based on the present disclosure. In that regard, FIG. 7B alsoillustrates a cross-sectional view of the device 100B at thisfabrication stage. The edge passivation process may employ techniquessimilar to the surface passivation as described earlier with respect tooperation 206. In a first embodiment of the edge passivation process, anultra-thin layer of aluminum is deposited (at least 0.5 nm) and issubsequently oxidized. In a second embodiment of the edge passivationprocess, an edge passivation layer of a dielectric material such asAl₂O₃ is deposited by ALD. Other embodiments of edge passivation arewithin the scope of the present disclosure.

In an embodiment as shown in FIGS. 7A and 7B, operation 208 results inmultiple BP active regions 101 over the substrate 102. Each BP activeregion 101 includes a portion of the BP layer 104 covered by a portionof the surface passivation layer 106 and portions of the edgepassivation layer 106A. The multiple BP active regions 101 may be spacedapart from each other by less than 20 nm, such as less than 5 nm. EachBP active region 101 may have a length L_(x) in the “x” directionranging from 10 nm to more than 1 micrometer, such as from 20 nm to 100nm, and have a width W_(y) in the “y” direction as small as severalnanometers, such as from 2 nm to 20 nm. Each BP active region 101 maytake the shape of a rectangle, or another geometric or polygonal shape.Further, the number of phosphorene layers in different active regions101 may be the same or different. For example, one BP active region 101may have two phosphorene layers, while another BP active region 101 mayhave three phosphorene layers. This provides improved design flexibilityas the number of stacked phosphorene layers in a BP active region 101determines the band gap of the BP active regions 101, which in turnaffects various electrical characteristics of the resultant transistorsuch as threshold voltage and on-state and off-state currents. Invarious embodiments, the number of phosphorene layers in two BP activeregions 101 may be the same or different. Further, each BP active region101 may include any number of phosphorene layers. In the followingdiscussion, FIGS. 1A, 1B, 1C, and 8-17 illustrate one transistor formedin one BP active region 101.

At operation 210, the method 200 (FIG. 2A) forms a gate stack 105 overthe passivation layer 106. Referring to FIG. 8, the gate stack 105 isformed over the passivation layer 106 and over the channel region 104 b.In the present embodiment, the gate stack 105 is a dummy gate stack andwill be replaced by a final gate stack 108 (FIGS. 1A and 1B). In anotherembodiment, the gate stack 105 is the final gate 108. To further thisembodiment, a portion of the passivation layer 106 underneath the gatestack 105 may be used as a gate dielectric layer.

In the present embodiment, the dummy gate stack 105 includes a dummyelectrode layer 111 over the passivation layer 106 and a hard mask layer113 over the dummy electrode layer 111. The dummy electrode layer 111may comprise poly-crystalline silicon (poly-Si) and may be formed bysuitable deposition processes such as low-pressure chemical vapordeposition (LPCVD) and plasma-enhanced CVD (PECVD). The thickness (alongthe “z” direction) of the dummy gate electrode 111 may range from 10 nmto 100 nm. An optional dummy gate dielectric layer (e.g., a layer ofsilicon oxide) may be deposited on the passivation layer 106 prior tothe deposition of the dummy gate electrode 111. The hard mask layer 113may include one or more layers of material such as silicon oxide and/orsilicon nitride and is formed by deposition processes. In an embodiment,after the layers of materials are deposited over the passivation layer106, one or more lithography and etching processes are performed to formthe dummy gate stack 105. A gate spacer 114 is optionally formedadjacent to the dummy gate stack 105. The gate spacer 114 may be formedby chemical vapor deposition of Si₃N₄ followed by reactive ion etching,in one example.

At operation 212, the method 200 (FIG. 2A) forms an inter-layerdielectric (ILD) layer 118 over the passivation layer 106 and the dummygate stack 105. Referring to FIG. 9, the ILD layer 118 may includematerials such as tetraethylorthosilicate (TEOS) oxide, un-dopedsilicate glass, or doped silicon oxide such as borophosphosilicate glass(BPSG), fused silica glass (FSG), phosphosilicate glass (PSG), borondoped silicon glass (BSG), and/or other suitable dielectric materials.The ILD layer 118 may be deposited by a PECVD process or other suitabledeposition technique. In an embodiment, the ILD layer 118 is formed by aflowable CVD (FCVD) process. The FCVD process includes depositing aflowable material (such as a liquid compound) over the substrate 102 tofill various trenches, and converting the flowable material to a solidmaterial by a suitable technique, such as thermal annealing orultra-violet radiation. The ILD layer 118 is then etched back orplanarized by a chemical mechanical planarization (CMP) process. The ILDlayer 118 has a thickness equal to or greater than the height of thedummy gate stack 105. In an embodiment, a contact etch stop layer (CESL)may be formed underneath the ILD layer 118. The CESL may include siliconnitride, silicon oxynitride, silicon nitride with oxygen (O) or carbon(C) elements, and/or other materials.

At operation 214, the method 200 (FIG. 2A) etches the ILD layer 118 toform openings (contact holes) 107. Referring to FIG. 10, the openings107 expose first portions 106′ of the passivation layer 106 over the S/Dregions 104 a. In embodiments, the openings 107 are formed by one ormore photolithography and etching processes. For example, thephotolithography process forms a hard mask and the device 100A/B is thenetched through the hard mask. The etching process may include a suitablewet etch, dry (plasma) etch, and/or other processes. For example, a dryetching process may use chlorine-containing gases, fluorine-containinggases, other etching gases, or a combination thereof. The wet etchingsolutions may include NH₄OH, HF (hydrofluoric acid) or diluted HF,deionized water, TMAH (tetramethylammonium hydroxide), other suitablewet etching solutions, or combinations thereof. The etching processstops at the passivation layer 106.

At operation 216, the method 200 (FIG. 2A) removes the first portions106′ of the passivation layer to expose the BP layer 104 underneath, asshown in FIG. 11. In the present embodiment, operation 216 is performedin an etch chamber of a multi-chamber tool, such as the chamber 308 ofthe tool 400 (FIG. 19). The etching process may be a wet etch or a dryetch. The wet etch may comprise etching in water (H₂O) or dilutedhydrofluoric acid (HF). The dry etch may be performed using anoxygen-containing plasma, or simply using water vapor. The reaction ratemay be adjusted by tuning the temperature, pressure, and oxygenconcentration. The etching process exposes top surfaces 104′ of the BPlayer 104 in the S/D regions 104 a.

At operation 218, the method 200 (FIG. 2B) deposits a contact material116 into the openings 107. Referring to FIG. 12, the contact material116 fills the openings 107 and comes in direct contact with the BP layer104 in the S/D regions 104 a, thereby forming S/D contacts 116. In thepresent embodiment, operation 218 is performed without breaking vacuumsubsequent to operation 216. In one example, operation 216 is performedin the chamber 308 of the tool 400 (FIG. 19). After the passivationlayer 106 is etched and the BP layer 104 is exposed, the device 100A/Bis transferred from the chamber 308 to the chamber 312 without breakingvacuum. This ensures that the top surfaces 104′ of the BP layer 104remain substantially free of oxidation. Subsequently, the contactmaterial 116 is deposited into the openings 107 in the chamber 312. Thismay be a chemical vapor deposition process or a physical vapordeposition process. The contact material 116 is metallic in nature, andmay be metallic nitrides, metallic or conductive oxides, elementalmetals, or combinations thereof. The elemental metals may be selectedfrom, but not limited to, the group consisting of Ti, V, Co, Ni, Zr, Mo,Tc, Rh, Pd, Hf, Ta, W, Re, Ir, and Pt. After the deposition of thecontact material 116, a CMP process is performed to planarize thesurface of the device 100A/B. In an embodiment, the FET 100A/B is ap-channel field-effect transistor, and the contact material 116 has awork function higher than about 4.5 eV. In another embodiment, the FET100A/B is an n-channel field-effect transistor, and the contact material116 has a work function lower than about 4.5 eV.

At operation 220, the method 200 (FIG. 2B) removes the dummy gate stack105. Referring to FIG. 13, the dummy gate stack 105, including the hardmask layer 113 and the dummy gate electrode 111, is removed and anopening 109 is formed. The opening 109 exposes a second portion 106″ ofthe passivation layer 106 over the channel region 104 b. The dummy gatestack 105 may be removed by one or more etching processes that mayinclude a wet etch, a dry etch, and/or other suitable etch techniques.The etching processes stop at the passivation layer 106.

At operation 222, the method 200 (FIG. 2B) removes the second portion106″ of the passivation layer 106 to expose the BP layer 104 underneath,as shown in FIG. 14. In the present embodiment, operation 222 isperformed in an etch chamber of a multi-chamber tool, such as thechamber 308 of the tool 400 (FIG. 19). This is similar to operation 216discussed above. After the passivation layer 106 is etched, a topsurface 104″ of the BP layer 104 is exposed in the opening 109.

In an embodiment, the method 200 proceeds to an optional operation 224(FIG. 2B) to reduce the thickness of the BP layer 104 in the channelregion 104 b. Referring to FIG. 15, the top surface 104″ of the BP layer104 is recessed in the opening 109. In embodiments, operation 224 isperformed without breaking vacuum subsequent to operation 222. In oneexample, operations 222 and 224 are performed in the chambers 308 and310 respectively (FIG. 19). After the passivation layer 106″ is etchedand the BP layer 104 is exposed, the device 100A/B is transferred fromthe chamber 308 to the chamber 310 without breaking vacuum.Subsequently, the BP layer 104 is etched in the opening 109 to recessthe top surface 104″. In an embodiment, the etching process is an atomiclayer etching (ALE). Operation 224 is optional and is not performed insome embodiments of the method 200. When performed, operation 224provides for improved design flexibility as different transistors in oneBP active region 101 (FIGS. 7A and 7B) can have different BP layerthicknesses (e.g., different numbers of phosphorene layers), whosebenefits have been discussed above with respect to FIG. 7B.

At operation 226, the method 200 (FIG. 2B) deposits a gate dielectriclayer 110 over the top surface 104″ in the opening 109. Referring toFIG. 16, the gate dielectric layer 110 is in direct contact with the BPlayer 104 in the channel region 104 b. In the present embodiment, thedevice 100A/B does not go through the optional operation 224, andoperation 226 is performed without breaking vacuum subsequent tooperation 222. In one example, operations 222 and 226 are performed inthe chambers 308 and 312 respectively (FIG. 19). After the passivationlayer 106″ (FIG. 13) is etched and the BP layer 104 is exposed (FIG.14), the device 100A/B is transferred from the chamber 308 to thechamber 312 without breaking vacuum. This ensures that the top surface104″ (FIG. 14) of the BP layer 104 remains substantially free ofoxidation. Subsequently, the gate dielectric layer 110 is deposited intothe openings 109 in the chamber 312. In an embodiment, the gatedielectric layer 110 is a high-permittivity (high-K) dielectric materialwith a relative dielectric constant of at least 10, such as 15 orgreater. The gate dielectric layer 110 may comprise hafnium oxide(HfO₂), zirconium oxide (ZrO₂), yttrium oxide (Y₂O₃), or other metaloxides.

At operation 228, the method 200 (FIG. 2B) forms a gate electrode 112over the gate dielectric layer 110. Referring to FIG. 17, the gateelectrode 112 may comprise one or more metals such as tantalum (Ta),titanium (Ti), tungsten (W), aluminum (Al), or combinations thereof invarious embodiments. The gate electrode 112 may also comprise nitridesor carbides of the various metals. In embodiments, the gate electrode112 may comprise multiple layers of various metals, such as a tungstenlayer on a gate work function layer on a barrier layer. The variouslayers may be formed by CVD, PVD, plating, and/or other suitableprocesses. A CMP process may be performed to remove excess materialsfrom the gate stack 108 and to planarize a top surface of the device100A/B. The final gate stack 108 includes the gate dielectric layer 110and the gate electrode 112.

At operation 230, the method 200 (FIG. 2B) proceeds to further steps tocomplete the fabrication of the device 100A/B. For example, operation230 may form a gate contact electrically connecting the gate stack 108,and may form metal interconnects connecting the S/D and gate contacts ofthe device 100A/B to other devices to form a complete IC. As anotherexample, the method 200 may form a plurality of transistors, wherein afirst subset of the transistors have thinner BP layer in the respectivechannel regions than a second subset of the transistors. In anembodiment for realizing such structure, the method 200 (FIG. 2B) may,after the operation 218 is done, perform the operations 220, 222, 226,and 228 to replace dummy gates in the second subset of transistors whilethe first subset of transistors are masked. Subsequently, the method 200performs the operations 220, 222, 224, 226, and 228 to replace dummygates in the first subset of transistors, thereby forming the firstsubset of transistors having reduced BP layer thickness in therespective channel regions.

In the present embodiment, the gate-replacement process (includingoperations 220-228) is performed after the S/D contact formation process(including operations 214-218). However, this is not limiting. In analternative embodiment, the gate-replacement process is performed beforethe S/D contact formation process. To further this embodiment,operations 220-228 may be performed after operation 212 and beforeoperation 214.

Although not intended to be limiting, one or more embodiments of thepresent disclosure provide many benefits to a semiconductor device andthe formation thereof. For example, embodiments of the presentdisclosure provide methods for manufacturing ultra-thin body transistorshaving a black phosphorus (BP) channel. Some embodiments of the methodsform a passivation layer, including surface and/or edge passivation,over a BP layer. The passivation layer prevents the BP layer from beingexposed to moisture and air during fabrication processes. When thepassivation layer is etched to expose the BP layer in order to form afeature (e.g., an S/D contact or a gate dielectric layer) in directcontact with the BP layer, the passivation etching process and thesubsequent deposition process are performed without breaking vacuum toensure the uniform surface property of the BP layer. Further,embodiments of the present disclosure provide cluster tools withmultiple chambers where a device with a surface/edge passivated BP layercan be manufactured. Further still, embodiments of the presentdisclosure provide methods of separating one large BP layer intomultiple smaller BP active regions with each BP active region covered bya surface/edge passivation. In various embodiments, since the surface ofthe BP layer remains substantially free of oxidation during thefabrication processes, the field-effect transistors employing the BPlayer as the ultra-thin channel have uniform and predictableperformance.

In one exemplary aspect, the present disclosure is directed to afield-effect transistor (FET). The FET includes a black phosphorus (BP)layer over a substrate. The BP layer includes a channel region, a sourceregion, and a drain region. The FET further includes a passivation layerover the BP layer and in direct contact with the BP layer. Thepassivation layer provides a first opening over the source region and asecond opening over the drain region. The FET further includes a sourcecontact in direct contact with the source region through the firstopening and a drain contact in direct contact with the drain regionthrough the second opening. The FET further includes a gate electrodeover the channel region. In an embodiment, the passivation layer furtherincludes a third opening over the channel region and the FET furtherincludes a gate dielectric layer in direct contact with the channelregion through the third opening, wherein the gate dielectric layer isbetween the channel region and the gate electrode.

In another exemplary aspect, the present disclosure is directed to anintegrated circuit (IC). The IC includes a first transistor having afirst gate dielectric layer overlying a first black phosphorus layerhaving a first thickness, and a second transistor having a second gatedielectric overlying a second black phosphorus layer having a secondthickness. The first black phosphorus layer includes a source region anda drain region underneath a first passivation layer. The second blackphosphorus layer includes a source region and a drain region underneatha second passivation layer. In an embodiment, the first thickness isdifferent from the second thickness. In an embodiment, the first andsecond passivation layers are different portions of same passivationlayer. In an embodiment, the first and second transistors reside infirst and second BP active regions of the IC. In a further embodiment,the first and second BP active regions each include edge passivation. Inanother embodiment, the first and second transistors reside in same BPactive region of the IC, while the first and second thicknesses aredifferent.

In another exemplary aspect, the present disclosure is directed to amethod of forming a field effect transistor (FET). The method includesforming a black phosphorus (BP) layer over a substrate and forming apassivation layer over the BP layer without breaking vacuum. The methodfurther includes forming a gate stack over the passivation layer andforming an inter-layer dielectric (ILD) layer over the passivation layerand the gate stack. The method further includes etching the ILD layer toform first openings thereby exposing first portions of the passivationlayer. The method further includes removing the first portions of thepassivation layer in the first openings to expose the BP layer anddepositing a metal in the first openings without breaking vacuum. In anembodiment, the gate stack is a dummy gate stack and the method furtherincludes replacing the dummy gate stack with a final gate stack.

In another exemplary aspect, the present disclosure is directed to amethod of forming a field effect transistor (FET). The method includesforming a black phosphorus (BP) layer over a substrate and forming apassivation layer over the BP layer without breaking vacuum. The methodfurther includes forming a dummy gate stack over the passivation layerand forming an inter-layer dielectric (ILD) layer over the passivationlayer and the dummy gate stack. The method further includes etching theILD layer to form first openings on opposing sides of the dummy gatestack, wherein the first openings expose first portions of thepassivation layer. The method further includes removing the firstportions of the passivation layer in the first openings to expose the BPlayer and depositing a metal in the first openings without breakingvacuum. The method further includes removing the dummy gate stack toform a second opening thereby exposing a second portion of thepassivation layer. The method further includes removing the secondportion of the passivation layer in the second opening to expose the BPlayer and depositing a gate dielectric layer over the BP layer in thesecond opening without breaking vacuum. The method further includesforming a gate electrode over the gate dielectric layer.

Another exemplary transistor includes a phosphorene-containing layerhaving a channel region, a source region, and a drain region definedtherein. A passivation layer is disposed over the phosphorene-containinglayer. The transistor further includes a source contact extendingthrough the passivation layer, such that the source contact is coupledwith the source region, and a drain contact extending through thepassivation layer, such that the drain contact is coupled with the drainregion. A gate stack is disposed over the channel region.

In some embodiments, the gate stack extends through the passivationlayer, and the gate stack includes a gate dielectric layer and a gateelectrode layer disposed over the gate dielectric layer, the gatedielectric extending through the passivation layer and contacting thechannel region. In some embodiments, the gate stack includes a gateelectrode layer disposed over the passivation layer, such that a portionof the passivation layer serves as a gate dielectric layer of the gatestack. In some embodiments, the passivation layer includes a metal oxidematerial, a dielectric material, or a semiconductor material. In someembodiments, a surface of the phosphorene-containing layer is free ofphosphorous oxide. In some embodiments, the phosphorene-containing layeris disposed over a planar surface of a substrate. In some embodiments,the phosphorene-containing layer is disposed over a non-planar surfaceof a substrate. In some embodiments, the substrate includes aninsulating material.

An exemplary integrated circuit device includes a transistor having agate stack disposed over a channel region. The channel region isdisposed between a source region and a drain region of the transistor.The channel region, the source region, and the drain region are definedin a phosphorene-containing layer disposed over a substrate. Apassivation layer is disposed over the source region and the drainregion defined in the phosphorene-containing layer. In some embodiments,the integrated circuit device further includes a source contact and adrain contact extending through the passivation layer, such that thesource contact is coupled with the source region and the drain contactis coupled with the drain region. In some embodiments, the substrateincludes an insulating-containing material layer over which thephosphorene-containing layer is disposed.

The foregoing outlines features of several embodiments so that those ofordinary skill in the art may better understand the aspects of thepresent disclosure. Those of ordinary skill in the art should appreciatethat they may readily use the present disclosure as a basis fordesigning or modifying other processes and structures for carrying outthe same purposes and/or achieving the same advantages of theembodiments introduced herein. Those of ordinary skill in the art shouldalso realize that such equivalent constructions do not depart from thespirit and scope of the present disclosure, and that they may makevarious changes, substitutions, and alterations herein without departingfrom the spirit and scope of the present disclosure.

What is claimed is:
 1. A method comprising: forming aphosphorene-containing layer over a substrate, wherein a channel region,a source region, and a drain region are defined in thephosphorene-containing layer, and further wherein the channel region isdisposed between a source region and a drain region; and forming apassivation layer over the source region and the drain region defined inthe phosphorene-containing layer.
 2. The method of claim 1, wherein theforming the passivation layer includes: depositing a metal-containinglayer over the phosphorene-containing layer; and oxidizing themetal-containing layer.
 3. The method of claim 2, wherein the oxidizingthe metal-containing layer includes exposing the metal-containing layerto air.
 4. The method of claim 1, wherein the forming the passivationlayer includes performing an atomic layer deposition process to deposita metal-and-oxygen-containing layer over the phosphorene-containinglayer.
 5. The method of claim 1, wherein the forming thephosphorene-containing layer and the forming the passivation layer areperformed without breaking vacuum.
 6. The method of claim 1, furthercomprising forming a gate structure over the channel region defined inthe phosphorene-containing layer, wherein the gate structure includes agate electrode disposed over the passivation layer, such that a portionof the passivation layer serves as a gate dielectric of the gatestructure.
 7. The method of claim 1, further comprising forming a gatestructure over the channel region defined in the phosphorene-containinglayer, wherein the gate structure includes a gate dielectric and a gateelectrode, the gate dielectric extending through the passivation layerand contacting the channel region.
 8. The method of claim 1, whereinforming the passivation layer includes forming a first passivation layerover a top surface of the phosphorene-containing layer and forming asecond passivation layer over side surfaces of thephosphorene-containing layer.
 9. The method of claim 1, wherein thesubstrate includes a fin structure, and further wherein thephosphorene-containing layer and the passivation layer are formed oversurfaces defining the fin structure.
 10. A method comprising: forming aphosphorene-containing layer over a substrate; forming a passivationlayer over the phosphorene-containing layer; forming a gate structureover the passivation layer, wherein the gate structure is positionedover a channel region defined in the phosphorene-containing layer;forming a source contact that extends through the passivation layer,such that the source contact is coupled with a source region defined inthe phosphorene-containing layer; and forming a drain contact thatextends through the passivation layer, such that the drain contact iscoupled with a drain region defined in the phosphorene-containing layer.11. The method of claim 10, wherein the forming the source contact andthe drain contact includes: depositing an interlayer dielectric layerover the passivation layer; etching the interlayer dielectric layer toform a first contact opening and a second contact opening thatrespectively expose a first portion and a second portion of thepassivation layer; etching the first portion and the second portion ofthe passivation layer to extend the first contact opening and the secondcontact opening, such that the first contact opening and the secondcontact opening respectively expose the source region and the drainregion defined in the phosphorene-containing layer; and filling thefirst contact opening and the second contact opening with ametal-containing material.
 12. The method of claim 10, wherein theforming the gate structure includes: forming a dummy gate of the gatestructure; and replacing the dummy gate with a metal gate.
 13. Themethod of claim 12, wherein the dummy gate is replaced with the metalgate after forming the source contact and the drain contact.
 14. Themethod of claim 12, wherein the dummy gate is replaced with the metalgate before forming the source contact and the drain contact.
 15. Themethod of claim 12, wherein the replacing the dummy gate with the metalgate includes: etching the dummy gate to form a gate opening in the gatestructure that exposes a portion of the passivation layer; etching theportion of the passivation layer to extend the gate opening, such thatgate opening exposes the channel region defined in thephosphorene-containing layer; and depositing a gate dielectric and agate electrode in the gate opening.
 16. The method of claim 15, whereinthe replacing the dummy gate with the metal gate further includesreducing a thickness of the phosphorene-containing layer exposed by thegate opening.
 17. A method comprising: depositing aphosphorene-containing layer; depositing a passivation layer over thephosphorene-containing layer; patterning the phosphorene-containinglayer and the passivation layer to define an active region thatcorresponds with a transistor, wherein the transistor includes a channelregion and source/drain regions defined in the phosphorene-containinglayer; forming a gate over a first portion of the passivation layerdisposed over the channel region defined in the phosphorene-containinglayer; depositing an interlayer dielectric layer over the passivationlayer; etching the interlayer dielectric layer to form first openings,thereby exposing second portions of the passivation layer; etching thesecond portions of the passivation layer to extend the first openings,thereby exposing the source/drain regions defined in thephosphorene-containing layer; and filling the first openings with aconductive material.
 18. The method of claim 17, wherein thephosphorene-containing layer is deposited over a dielectric-containinglayer.
 19. The method of claim 17, wherein the phosphorene-containinglayer and the passivation layer are deposited over a fin structure. 20.The method of claim 17, wherein the passivation layer is a firstpassivation layer, the method further comprising, after patterning thephosphorene-containing layer and the passivation layer, forming a secondpassivation layer over portions of the phosphorene-containing layer notcovered by the first passivation layer.